The following invention relates to composite structures and, more particularly, to a uniquely configured composite torque tube end fitting design that is specifically adapted to provide increased resistance to axial and torque loads.
The prior art includes many structural configurations wherein torque from one member is transferred to another member. For example, an automobile driveshaft is configured to transmit torque between an engine and driving wheels. Similarly, driveshafts may be incorporated into various types of aircraft and seagoing ships. Driveshafts are generally comprised of elongate rod or tubing having end fittings specifically configured to be connected to transmission devices.
With an increasing demand for more efficient structures, there has been a trend away from metallic structures in favor of composite structures. In the quest for lightweight structures that can efficiently transmit static and dynamic loads, composites possess many favorable characteristics. For example, a composite driveshaft may have a weight reduction of one-half to one-quarter the weight of a conventional metallic driveshaft of the type that is commonly used in motor vehicles. Other benefits of using composites in driveshafts lies in improved mechanical properties including improved modulus of velocity, density, flexural rigidity, and/or torsional rigidity.
Advantageously, composites also allow for fine tuning of certain ones of these mechanical properties for a given application by merely changing the arrangement of the basic elements. For example, by changing the angle of orientation of load-carrying fibers embedded in the composite material, the stiffness of a torque-carrying composite assembly can be altered. The capability to tune such mechanical characteristics is especially important in aerospace applications wherein weight reduction is a critical factor. Dynamic loading applications for torque tube assemblies includes control rods, torque rods, torque shafts. Static loading applications for torque tube assemblies includes numerous structural elements such as aircraft wing struts.
Prior art composite torque tube designs rely upon circular end fittings that are inserted into a free, open end of a composite laminate torque member. An adhesive bond may be provided between the end fitting and the composite member in order to improve the torque carrying capability of the torque tube. Unfortunately, the greater the amount of axial or torque loads that is applied to the torque tube, the larger the amount of surface area that is required in order to accommodate such higher loads. As a result, applications of extremely high axial or torque loads necessitate the use of end fittings that extend along a large portion of the composite member. Unfortunately, the end fittings are increased in length merely for the purpose of increasing the amount of available adhesive bond area between the end fitting and the composite member.
In extreme circumstances, and depending upon the length of the composite member, the amount of adhesive bond area required in order to adequately transmit torque loads necessitates that end fitting on either end of the composite tube eventually extends along its entire length. Such extreme lengths of the end fittings inside the composite member negates any benefit gained through the use of the lightweight composite materials in torque tube assemblies. The resulting structure may be more time-consume and costly to manufacture that a metallic version constructed in accordance with prior art design practices.
Attempts to increase the torque and axial load-carrying capability of composite torque tube designs included the use of a pin or fastener inserted radially or diametrically through the composite tube member and the end fitting. However, as is well known in the art of composite construction, forming a hole in a composite structure oftentimes results in severing one or more of the load carrying fibers within the composite tube. Furthermore, the act of forming a hole in the composite member is sometimes expensive and time consuming due to the need for specialized drilling equipment to prevent breaking or severing the resin and fiber elements around the hole. Finally, the addition of a pin or fastener to aid in transmitting a portion of the torque from the end fitting to the tube member is not an optimized lightweight solution.
Also included in the prior art are other attempts for improving transmissibility between end fittings and the composite tube member into which the end fitting is inserted. For example, special surface features may be provided on an outer surface of the end fitting in order to increase the engagable surface area between the end fitting and the interior of the composite tube. For example, serrations added to the end fitting may allow the end fitting to grasp the inner surface of the composite tube member in order to better transfer load. However, the need to form special surface features such as serrations on the outer surface of the end fitting presents additional machining challenges and increases fabrication costs. Furthermore, the serrations and surface features themselves may sever reinforcing fibers embedded within the resin matrix of the composite tube member thereby increasing the risk of a failure in the transmission of high torsional loads.
As can be seen, there exists a need in the art for a composite torque tube end fitting design that efficiently captures the end fitting within a composite tube and locks the end fitting inside the composite tube member using mechanical advantage. Furthermore, there exists a need in the art for a composite torque tube design that prevents relative rotation between the end fitting and the torque tube member during the application of torsional loads. Additionally, there exists a need in the art for composite torque tube design having axial loading capability in addition to torsional loading capability. Finally, there exists a need in the art for composite torque tube design that is of some simple construction and of low cost and which may be formed using simple manufacturing methods.